The present invention relates to a motor speed control circuit, and more particularly to a circuit for maintaining the speed of a motor at a constant level with a high degree of accuracy.
Precision motor speed control circuits have a variety of applications, including phonographic turntables, magnetic audio and video tape recorders, and disk drive units.
One circuit for accurately controlling the speed of a motor is disclosed in U.S. Pat. No. 4,240,014--Muller, assigned to Papst-Motoren KG. In the operation of this ciruit, trigger pulses are generated during the rotation of the motor, for example by an electro-optical encoder or a magnetic tacho-generator. With each detected pulse, a monostable (one-shot) multivibrator is triggered to generate a command pulse having a precisely controlled, predetermined duration. At the end of this command pulse, a capacitor is quickly discharged and then recharged until the next trigger pulse from the motor is detected. The level of charge on the capacitor at this point is maintained for the duration of the succeeding command pulse from the monostable multivibrator. This charge is used as an input to an amplifier that controls the amount of current applied to the motor.
Thus, when the motor is operating at a speed below that which is desired, a relatively long period of time will elapse between the end of one command pulse from the monostable multivibrator and the detection of the next trigger pulse from the motor, causing the capacitor to be charged to a relatively high level. Consequently, a large amount of current will be applied to the motor to cause it to speed up. On the other hand, when the motor is operating at a relatively fast speed, the capacitor will not be charged to as great an extent. Therefore, less current will be applied to the motor and it will slow down.
The type of motor control circuit exemplified in the previously mentioned Papst-Motoren patent is in relatively widespread use because it is able to regulate motor speed with a good deal of accuracy, e.g. within one percent of the desired speed. However, it is desirable to improve upon certain aspects of this circuit. More particularly, since the length of the timing pulse generated by the monostable multivibrator must be very precise, highly accurate components are required. Specifically, a high quality timing capacitor that charges at a well defined rate is used to determine the length of the timing pulses. In addition, a temperature compensation resistor is required to insure that the current that charges the timing capacitor is insensitive to temperature variations. The requirement for the high quality compensate and the need to compensate for temperature adds to the cost and complexity of the circuit.
In addition, the motor control circuit of the patent operates entirely in an analog mode, which also adds to its complexity. It is desirable to utilize present-day digital techniques in the motor control circuit to the extent possible because of the high degree of reliability that they afford at a relatively low cost.